1. Field of the Invention
The present invention relates to a steering system for rear wheels of a vehicle that controls a toe angle depending on steering angle and steering angle speed of a steering wheel of the vehicle.
2. Description of the Related Art
Conventionally, in a vehicle such as a four-wheel vehicle, rear wheels are controlled to be orientated outward (toe-out) or inward (toe-in) relative to a vehicle traveling direction in a top plan view, so as to enhance movement property of a vehicle in accordance with a driver's driving operation.
For example, in order to cancel a camber angle, both rear wheels are controlled in a toe-in orientation so as to improve straight line vehicle stability, or both rear wheels are also controlled in a toe-in orientation at the time of braking so as to enhance driving stability of a vehicle.
In such a rear-toe control, it is possible to enhance movement performance of a vehicle by controlling a steering angle of rear wheels in accordance with a steering angle and a steering angle speed of a steering wheel handled by a driver.    [Patent Document 1] JP 05-330444 A
By the way, in the case in which a steering angle of rear wheels is controlled in accordance with a steering angle and or steering angle speed of a steering wheel handled by a driver, if a control gain is increased, even very small swings of the steering wheel to the right and the left cause the rear wheels to be steered, so that the vehicle moves too sensitively, which may cause unpleasant feeling to the driver.
To counter such a phenomenon, if a dead zone is provided in the vicinity of a neutral position, i.e. a zero position of a steering angle of a steering wheel; i.e. in vicinity of the neutral position thereof, the rear wheels are not steered when driving straightly, which eliminates the above unpleasant feeling, but when the steering angle of the steering wheel passes through the neutral position at the time of turning back the steering wheel significantly, movements of the rear wheels are suddenly stopped, which also causes another unpleasant feeling.
The patent document 1 discloses a technique of 4WS (Wheels Steering) using a dead zone.
To address the above problems, the present invention has an object to provide a steering system for rear wheels of a vehicle, free from effects from a dead zone in the vicinity of a zero steering angle of a steering wheel, causing no unpleasant feeling to a driver during driving.